The present invention relates to a method for the preparation of a lanthanum manganite powder which is useful as a material of various kinds of catalysts, electrodes and the like mainly in the form of a sintered body.
It is conventional that a powder of lanthanum manganite, which is sometimes partially substituted by strontium to have a composition formula of (La.sub.1-x Sr.sub.x).sub.y MnO.sub.z, in which x is 0 or a positive number not exceeding 0.5, y is a positive number in the range from 0.8 to 1 and z is a positive number not exceeding 3, is prepared by blending powders of each of a specified and weighed amount of lanthanum oxide, manganese carbonate and, optionally, strontium carbonate by a dry process, wet process or a combination of both and calcining the powder blend at a temperature of 1200.degree. C. or higher.
The above described conventional method for the preparation of a lanthanum manganite powder has problems in several respects. For example, it is almost unavoidable that a considerable portion of the starting powders remains unreacted in the product obtained by calcination and the amount of the unreacted starting materials can hardly be undetectably small even by conducting the process of calcination at a higher temperature for a longer length of time than usually undertaken. When a lanthanum manganite powder contains a substantial amount of unreacted lanthanum oxide, the sintered body prepared therefrom has a greatly decreased mechanical strength or the electric properties of the sintered body are very adversely affected. Disadvantages are also unavoidable by the high temperature calcination even when the lanthanum manganite powder is to be used in the form of a powder as such as in the applications for thermal spraying and the like because, even by setting aside the costs for the calcination at such a high temperature for a long time, growth of particles is unavoidable in the powder blend to give a semi-sintered mass of coarse particles which must be disintegrated and finely pulverized.
As a remedy for the above described disadvantages, Japanese Patent Kokai 4-74721 proposes a method for the preparation of a strontium-substituted lanthanum manganite powder in which an aqueous solution of water-soluble compounds of lanthanum, strontium and manganese in a specified proportion is admixed with an aqueous solution of ammonium carbonate to precipitate a composite carbonate and the precipitates are collected by filtration, dried and calcined to give a lanthanum manganite powder. A problem in the lanthanum manganite powder obtained by this method is that, although the amount of the unreacted starting materials in the thus produced powder can be small enough, the product powder is poor in respect of the flowability behavior so that the powder is not suitable for use, for example, in thermal spraying in which good flowability of the refractory powder is essential, for example, in order not to cause bridging of the powder in the feeder hopper.